1. Field of the Invention
This invention relates to an integrated circuit and more particularly to a device and method for identifying a set of masks used in manufacture of the integrated circuit.
2. Description of the Relevant Art
A mask is a well known tool for placing a patterned image onto a semiconductor substrate. A mask is capable of transferring the image to an entire wafer or to another mask in a single exposure. As defined herein below, "mask" includes the well known term "reticle". Instead of transferring an entire wafer image in a single exposure, a reticle can be used to perform the transfer in a step-and-repeat fashion. In either instance, mask transfer or reticle transfer involves in a broad context the transfer of a patterned image from the mask (defined to include a reticle) to the wafer in a series of photolithography steps.
A mask is fabricated from a glass or glass-like (i.e., quartz) starting material. Upon the glass material, a coating of chrome can be placed. The chrome is then exposed and etched at select regions to form a pattern across the mask. The pattern consists of areas of transparent and opaque regions--the opaque regions being impermeable to radiation.
In order to form an integrated circuit upon a semiconductor substrate, multiple masks are necessary. Each mask is used to pattern a layer on the substrate, and successive layers are required to achieve an integrated circuit. In CMOS integrated circuit manufacture, for example, more than ten (and sometimes more than fifteen) mask layers are needed. The pattern on each mask is transferred in proper sequence to the upper surface of the semiconductor substrate. It is therefore the mask which determines the outcome (structure and function) of the resulting integrated circuit. Any change in the pattern of one or more masks will cause a resulting change in the circuit outcome.
During the development of an integrated circuit, it is oftentimes necessary to revise one or more masks to enhance integrated circuit performance. On large area integrated circuits or circuits which are highly dense and complex in functionality, the number of mask revisions are generally numerous. For example, a microprocessor integrated circuit may undergo a dozen or more revisions, entailing possible changes to a half-dozen or more layers for each revision. Keeping track of changes to each layer and associated revision numbers for a particular integrated circuit product can be unduly burdensome. Many manufacturers indicate mask revisions by changing the designator on the mask being revised. The designator is therefore patterned onto the substrate for visual detection by the end user. The user can therefore inspect the resulting wafer and determine the specific mask revision number used. For example, a wafer which embodies the second revision to n-type well, the third revision to polysilicon, the fourth revision to first metal and the fifth revision to second metal has on its upper surface those revision numbers etched from mask for visual inspection by the manufacturer. The manufacturer can then dice the wafer and package each die in accordance with the marked revision numbers.
Oftentimes, all that distinguishes an operable low-speed device from an operable high-speed device is a revison to one or more masks layers. While a manufacturer can determine the operability of a device based on the descriptors etched on the wafer surface, customers or end-users are not so fortunate. After the manufacturer sorts product according to mask layer revision, and places die into hermetically sealed packages, the die may no longer contain the descriptors if the descriptors are placed, for example, in scribe areas or test locations on a wafer. More importantly, once die are packaged, inspection of mask layer revision requires that the package be opened so that the user can view the descriptor if the discriptors are placed on the die or on the changed (revised) circuit area. Once the package is opened, it generally cannot be re-sealed. Therefore, the die associated with that package must be discarded. Not only is visual inspection of packaged die destructive to the die (i.e., forbids re-use of the die), but it is also burdensome and time consuming. Visual inspection requires tools for opening the package as well as an optical magnification device.
Visual inspection, especially visual inspection after packaging, presents numerous problems to the end-user. If an integrated circuit is sold as having a particular application based on a set of mask layers, and that a separate application can be achieved if a revision to the mask layer is used, then it would be advantageous for an end-user to know of the various applications and mask layer revisions associated therewith. For example, it may be necessary to signify various operating speeds of a integrated circuit device via revisions to mask layers. An updated mask layer could be used to produce a speed grade of a device that was previously unavailable. Thus, device operating modes can be mask programmable at various multiple clocking speeds through enable/disable selection internal or external to the device.
If the user, for example, is unable to achieve a system outcome based upon the inadequacy of a packaged integrated circuit, the user might wish to know how that integrated circuit can be modified to achieve system adequacy. In that instance, it would be advantageous for the user to contact the integrated circuit manufacturer and explain the encountered problem. If visual inspection is the only methodology for determining mask revision of that integrated circuit, then the manufacturer has little recourse in fixing the problem on that specific integrated circuit. If the integrated circuit is inadequate for reasons of speed, the manufacturer is unable to determine if the user-purchased integrated circuit is of the higher speed variety or the lower speed variety. The manufacturer therefore, cannot recommend substituting the integrated circuit for a higher speed (different mask set revision) version.
From both the user's and manufacturer's perspective, non-destructive identification of an after-market (packaged die incorporated into the user's system) integrated circuit would be highly desirable. Capability of non-destructive, electrical read-out on a package pin-out of the mask set used for forming the integrated circuit would be advantageous. Further, read-out must accommodate an infinite number of mask layers as well as an infinite number of mask layer revisions.